1. Field of the Invention
This invention relates generally to an adjustable magnetic system for producing a desired magnetic field strength and configuration, and more specifically, this invention relates to a magnetic system to produce a desired magnetic field strength and configuration for a nuclear magnetic resonance diagnostic device.
2. Description of the Prior Art
Although nuclear magnetic resonance (NMR) techniques have been known and utilized for several decades in analyzing various types of materials, it is only within the last few years that extensive efforts have resulted in the use of NMR techniques for analyzing various parts of the human body. As a diagnostic tool, NMR is a valuable supplement to currently utilized X-ray and ultrasonic non-invasive techniques. In addition, since NMR can provide much greater information about soft tissue than either X-ray or ultrasonic scanning, it has the capability of replacing invasive diagnostic techniques ranging from the consumption of radioactive materials to surgical incisions. Further, since bone does not provide an impediment to magnetic fields, there are certain imaging tasks that NMR can perform, such as looking at the spinal cord or cartilage inside the vertebrae, that cannot be accomplished with X-ray or ultrasonic techniques and which may be extremely dangerous or even impossible for invasive techniques. On top of these advantages, magnetic fields produce no known adverse effects in the body, although the extent to which field strength can be increased without producing such adverse effects may be questioned.
NMR diagnosis is achieved by placing the patient (or other object being analyzed) in a uniform relatively strong fixed magnetic field. The required strength of this field has not been adequately established as of this time. It appears that three kilogauss or less will produce very good imaging for the hydroxyl group. However, certain significant elements in body tissue, such as phosphorus, may require significantly stronger fields, although it appears that the great majority of metabollic states and functions that can be examined by NMR techniques do not require these very high field strengths.
The purpose of the relatively strong magnetic field is to produce an alignment of the atomic nuclei of the element of the body being analyzed. Due to the large amount of water in the body, much information can be obtained by observing the hydrogen nucleus or proton. Thus, it is the hydroxyl group that is of most practical benefit in NMR diagnostic techniques.
After the relatively large fixed magnetic field has established an alignment of the atomic nuclei, a radio frequency magnetic field (RF field) perpendicular to the fixed field is applied. This RF field effects the orientation of the atomic nuclei established by the relatively strong fixed magnetic field, the extent of which depends upon the particular element being observed. The RF field is then removed and the reaction of the atomic nuclei produces a magnetic effect that is picked up and analyzed to yield the desired information. Analysis of the magnetic signals is achieved by use of image processing computer technology largely derived from programs and equipment used in the well-known CAT (computerized axial tomography) scanners, which utilize X-ray beams to develop cross-sectional views of the body.
At the present time, most NMR diagnostic imaging devices have utilized solenoidal superconducting magnets for the relatively large constant magnetic field, which must provide a considerable degree of homogeneity across the area in question. The superconducting magnets utilize electrical currents circulating in coils formed of special metal alloys that display virtually no electrical resistance when maintained at temperatures near absolute zero. As a result, very large electrical currents may be passed through the coils, with the production of attendant very large magnetic fields across an air gap sufficiently large to hold a human body.
While the solenoidal superconducting magnets can provide the necessary magnetic field strength for successful NMR imaging, it is difficult and time consuming to obtain and maintain the necessary field uniformity, as well as being difficult and expensive to contain and minimize the undesired relatively large "fringe fields" produced by these magnets. These extraneous or fringe fields produced by the solenoidal superconducting magnets can be over 30 feet in diameter and more than 120 feet long. These large fringe fields are potentially dangerous to patients with pacemakers, metal implants or surgical clips; and these fields can interfere with the operation of computers and other electronic equipment. Also, since variations in the fringe field will affect the field in the diagnostic area, automobiles, elevators, trash bins or even a metal file cabinet within 60 feet of the unit can distort the magnetic field in the NMR device. Such distortion requires that the magnetic system be re-calibrated or re-shimmed to provide the necessary uniformity or homogeneity of the magnetic field in the diagnostic area. The delicate nature of the balance of forces that produces field homogeneity makes the superconducting NMR device particularly susceptible to damage or to distortion requiring re-calibration in the event of vibrations or shocks to the equipment.
An additional problem with superconducting magnets is that the conducting coils must be maintained near absolute zero (approximately -460.degree. Fahrenheit) in order to retain the superconducting characteristics. This requires the provision of equipment and procedures for inserting liquid nitrogen and/or helium and handling the resulting gases that boil off. The annual cost of the liquid helium and/or liquid nitrogen in a typical NMR installation will run from $50,000 to $100,000 or more. Thus, superconducting magnets involve not only the technical difficulties of maintaining cryogenic temperatures, but they also involve an on-going relatively high expense factor.
Various attempts have been made to overcome the problems associated with superconducting magnet NMR diagnostic devices. In some cases, a separate building is constructed of wood and other non-magnetic materials and is located in an area which can be isolated from the hazards and problems referred to above. Such a structure can easily run into costs in the range of $500,000 to $1,000,000. Attempts to avoid the cost and space factors involved in such a separate structure have included surrounding the NMR area with one-inch thick steel plates, or by placing the entire NMR magnet assembly in a thick steel pipe 10 to 15 feet in diameter.
In view of the difficulties associated with superconducting magnetic assemblies, other alternatives have been explored. Electromagnets (commonly referred to as "resistive") have been tried, but it has been found that these electromagnets require very careful temperature control. In addition, they consume very large amounts of electrical energy and can generate electrical "noise" which adversely affects image quality.
Permanent magnets have many desirable attributes for NMR applications, but until recently it was thought that permanent magnets were too weak and uncontrollable for serious consideration. However, a working NMR diagnostic device has been developed utilizing a permanent magnet. Unfortunately, in order to get the requisite field strength, this system weighs on the order of 100 tons. Such a very heavy weight creates housing and support problems, with the attendant large costs. In addition, the problems associated with achieving the desired magnetic field homogeneity, while yet providing flexibility for varying circumstances, still exist.